on same computer, second install with same key

On each of my PCs with XP I have XP installed twice with the same license. I
have a main OS for normal use and a second one with nothing extra installed
used to do backups. This is possible because Windows validates using the
first hard disk partition found on the physical computer, and no matter how
many separate installs on the same computer, validation is not a problem
using the same key. This is true if your copy of Windows has a key. Many OEMs
do not provide a key, but instead provide restore CDs. Though a key can be
discovered, a second install on the same machine will not activate with the
discovered key. One must clone the install, load registry hives on the clone,
search and replace drive letters, and scan all files in the install. I need
to know if Vista will work the same as XP in this regard. If so I will buy
one copy for one of my computers. Thanks